The packing density of digital signals, or information, is often subjected to temporal fluctuations during transmission and/or record and replay. This also is applicable to, for example, video signals which were processed with a source coder for irrelevancy and redundancy reduction. Designing the transmission route, or the recording medium, for the maximum packing density is technically expensive and uneconomic. The packing density can be held to an average value through temporal compression and subsequent expansion. However, variable word lengths of the data segments can result, depending upon the packing density. This creates a problem in recognizing data segments of differing lengths and in allocating the correct position of the blocks in the output picture. Data segment recognition is particularly difficult if interference removes the recognition criteria. If such interference occurs, it is possible that an accurate recognition can not be made, even after the interference has died out. A similar problem occurs in a digital video recorder when operating in a search mode where data can only be read piece by piece in short segments. It is desirable to reconstruct a low quality picture from data distorted by interference resulting from relatively short segments read during a search operating mode.
The invention is directed to an improved processing method of digital signals, for example a television picture, such that it may be decoded with sufficient quality during periods of interference such as that resulting from a medium transduced at a speed other than the recorded speed.
U.S. Pat. No. 4,907,101 teaches the batch-bulk recording of digital signals which have been received segment-wise. Data blocks having data segments with a smaller than average word length are filled with parts of the data segments from blocks having a larger than average word length. The data can, for example, consist of the coefficients of a discrete cosine transformation DCT. The direct or DC component and one, or several, important alternating or AC components and a terminating end-of-block characteristic or flag are first recorded for each n.times.m picture element block, typically n=8 and m=8. When the data segment length of the coded block is smaller than an average data segment length, the remaining available space is filled up with alternating components of a block which requires a larger than average data segment length and, if it fits, with the respective end-of-block characteristic. Since the blocks are of equal length, the start of each block will be at equal time intervals which aids the location during playback in the presence of errors.
In a conventional helical-scan recording system operating in the search mode, the reading heads or transducers, move over the recorded tracks at an angle different from that in the normal reproduction mode of operation with the consequence that tracks are only partially or batch-bulk read. Thus the reproduced signal is interrupted with bursts of noise or interference as the transducer crosses from track to track. Even when the reading head is mounted on actuators to provide accurate track following adjustment, there are search speeds where the recorded tracks will be only partially or batch-bulk read.
The problem arising from reading different tracks results from the fact that the tracks are written sequentially, thus there may be time or temporal differences between the information contained in each track. Hence in search, when the transducer crosses and reads multiple tracks, the recovered signal may comprise information representing different events in time, thus the recovered signal may exhibit temporal segmentation. For example, a recording of a stationary television image may not reveal temporal segmentation until there is motion within the scene whereupon the moving image parts will appear segmented. Hence the problem is the spatial positioning of the recovered segment of data blocks in the complete decoded signal, or picture. The solution is to insert location information at particular points in the signal to be recorded which is utilized during replay, to locate the signal components in accordance with the location information. The end-of-block characteristic appears to be most suitable for this purpose. However, if location information is added to every end-of-block characteristic the amount of data to be recorded is markedly increased.
Another possibility is the combination of several blocks, for example eight, into a larger block, hereinafter called a superblock. The superblock represents a portion of the picture and is provided with an address to locate it within the picture. This address may be positioned at the start or end of the superblock. The data rate is only negligibly increased by the recording of a single address for each superblock and the total coder efficiency is increased. The output signal is reassembled from the almost equally spaced (in time) decoded superblocks. The signal is segmented according to the size of the superblock and can have an unchanged spatial resolution.